1. Field of the Invention
The present invention relates to a measurement method for measuring a shape of a target from an interference pattern generated by interference between reflected light of the target and a reference spherical surface without making physical contact, and more particularly to a measurement method for measuring a shape of a gull wing type target.
2. Description of the Related Art
Recently, since an aspherical surface lens is adopted to an optical system such as a camera lens, the number of the lenses is reduced (reduction of the cost), the aberration is lowered, and the lens group is miniaturized. Therefore, the measurement of the aspherical surface shape is necessary, and a variety of measurement methods for measuring the aspherical surface have been proposed.
The measurement method for measuring the aspherical surface shape is roughly categorized into two. A first measurement method is a contact type method, which is a method of tracing a target surface by a probe. This method has small limitations for the shape, and has an advantage that a target which has a large amount of an aspherical surface or a complicated shape which does not have the symmetry can be comparatively easily measured. On the other hand, there is a disadvantage that it takes long measurement time and it possibly damages the target because of physically tracing the points.
A second method is a non-contact type method, and an interference measurement using light is typical. There are advantages in the non-contact type method that it takes comparatively short measurement time because the range radiated by the light can be collectively measured and that the target is not be damaged because of the non-contact type method. On the other hand, there is a disadvantage that the measurement of the target having a complicated shape is difficult.
According to a spherical surface interference method that is one example of non-contact type method, the difference between the reference wave surface and the shape of the target can be measured by analyzing an interference pattern obtained by the interference between the reference wave surface from a transmissive spherical surface made with high accuracy and the target. Since the spherical surface interference method can change the radius of curvature of the reference wave surface by changing the relative distance between the transmissive spherical surface and the target, it has characteristics that a lot of targets can be measured using one transmissive spherical surface.
When the target has an aspherical surface, it can not be easily measured unlike the case for the spherical surface. In this point, as a method for extending the spherical surface interference method to the aspherical surface, there are, for example, U.S. Pat. No. 6,781,700 and Japanese Patent Laid-Open No. 2004-125768.
U.S. Pat. No. 6,781,700 discloses a measurement method of an aspherical surface shape that has an axially symmetric design value. When the target having the axially symmetric aspherical surface shape is scanned in an optical axis direction, an annular-shaped interference pattern appears in a certain region. The shape of the target is measured by measuring the annular-shaped interference pattern more than once to mathematically calculate the shape of the target.
Japanese Patent Laid-Open No. 2004-125768 discloses that a target is divided into a plurality of partial regions to measure each of the regions. The measured partial regions are joined to measure the whole shape of the target. Hereinafter, in the specification, joining the partial regions is referred to as stitching. An interferometer is used for measuring each of the partial regions. In this case, the measurement of the shape is performed while tilting the target to be virtually approximated to an aspherical surface.
Although both U.S. Pat. No. 6,781,700 and Japanese Patent Laid-Open No. 2004-125768 measure the target having an aspherical surface, the aspherical surface shape is limited to either one of a convex shape and a concave shape. When a target is a shape where the convex shape and the concave shape are mixed such as a gull wing type shape (a free-form surface), the whole shape of the target can not be measured. Although such a target can be comparatively easily measured by using a contact type method, it is difficult to measure it by using a non-contact type method.